1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising a photoconductive layer of a selenium alloy and a protective layer formed thereon containing an anti-oxidizing agent, and an electrophotographic copying process and an apparatus using the particular electrophographic photoconductor.
2. Discussion of Background
Conventionally, a variety of electrophotographic photoconductors are known. For instance, there are known an electrophotographic photoconductor in which a photoconductive layer consisting essentially of selenium or a selenium alloy is formed on an electroconductive support; an electrophotographic photoconductor prepared by dispersing an inorganic photoconductive material, such as zinc oxide or cadmium sulfide, in a binder agent and coating the dispersion on an electroconductive support; and an electrophotographic photoconductor comprising a photoconductive layer which contains an organic photoconductive material such as a mixture of poly-N-vinylcarbazole and trinitrofluorenone, an azo pigment or amorphous silicon.
Recently a demand for an electrophotographic photoconductor having high reliability, capable of producing high quality images for a long period of time is increasing. In the case of an electrophotographic photoconductor with its photoconductive layer unprotected and exposed, the photoconductive layer is gradually damaged by corona charges applied thereto in the course of a charging process. Furthermore the photoconductive layer is physically and chemically deteriorated in a copying process while it is brought into contact with other members of an electrophotographic copying apparatus. There are the main factors for shortening the life of the electrophotographic photoconductor.
To solve the above-mentioned problem, methods of covering the surface of an electrophotographic photoconductor with a protective layer are known. More specifically, there are disclosed a method of forming an organic film on the surface of a photoconductive layer of an electrophotographic photoconductor in Japanese Patent Publication 38-015446; a method of providing an inorganic oxide layer on the surface of a photoconductive layer in Japanese Patent Publication 43-014517; a method of successively overlaying an adhesive layer and an insulating layer on a photoconductive layer in Japanese Patent Publication 43-027591; and methods of laminating an amorphous silicon (a-Si) layer, a-Si:N:H layer or a-Si:O:H layer on a photoconductive layer by the plasma CVD or the photo CVD in Japanese Laid-Open Patent Applications 57-179859 and 59-058437.
However, when the above-mentioned protective layers have a resistivity of 10.sup.14 .OMEGA..multidot.cm or more, which is considered to be too high in electrophotography, the residual potential of the photoconductor increases while in use, and the residual electric charges are gradually accumulated during the repetition of copying operation, which will hinder the practical operation of the photoconductor.
In order to cover the above-mentioned shortcoming of the protective layer, there is proposed in Japanese Patent Publication 52-024414 a method of optimizing the resistivity of a protective layer by adjusting the composition of a resin contained in the protective layer. Furthermore, methods of forming a photoconductive protective layer on a photoconductive layer are proposed, as disclosed in Japanese Patent Publications 48-038427, 43-016198 and 49-010258, and U.S. Pat. No. 2,901,348. In addition, there are disclosed a method of adding to a protective layer sensitizers such as dyes and charge transporting agents represented by Lewis acids, as in Japanese Patent Publication 44-000834 and Japanese Laid-Open Patent Application 53-133444; and a method of controlling the resistivity of a protective layer by adding finely-divided particles of metals or metallic oxides, as in Japanese Laid-Open Patent Application 53-003338.
When the particles of metals or metallic oxides are added to the protective layer, projected light for image formation is partially absorbed in the protective layer while passing therethrough. As a result, the amount of the light which reaches the photoconductive layer is decreased and accordingly the photosensitivity of the photoconductor is disadvantageously decreased.
To eliminate the above-mentioned disadvantage, there is further proposed in Japanese Laid-Open Patent Application 57-030546 a method of making a protective layer which is substantially transparent to visible light by dispersing in a protective layer metallic oxide particles having an average particle diameter of 0.3 .mu.m or less, which serve as a resistivity-controlling agent.
In the photoconductor provided with the above-mentioned protective layer, the reduction in the photosensitivity can be minimized, and the mechanical strength of the protective layer can be increased so that the resistance to wear can be remarkably improved.
However, it is found that the above-mentioned photoconductor has a problem that image flow occurs. Namely, blurred images are formed when the photoconductor is used repeatedly in a copying machine for an extended period of time under the conditions of high humidities or in the atmosphere where the ambient humidity drastically increases. The cause of such a phenomenon has not yet been clarified, but it is supposed that a resin contained in the protective layer is oxidized and deteriorated by ozone or various ions which are generated by corona charges applied to the photoconductor while it is repeatedly used. As a result, the resin is fractured or some radicals are formed. In addition to the above, the ozone and ions generated by the corona discharging of the photoconductor react with water and impurities such as a carbon dioxide gas in the air, so that nitrogen compounds and hydrophilic compounds containing carboxyl groups and aldehyde groups are formed. Those compounds are chemically adsorbed by deteriorated portions at the surface of the protective layer. When the photoconductor is operated under the conditions of high humidities or drastically increasing humidities, the protective layer of the photoconductor adsorbs a large amount of moisture, and the resistivity of the surface of the photoconductor is so much decreased that the image flow problem will occur in the photoconductor.
Furthermore, positively chargeable electrophotographic photoconductors comprising a charge transport layer, a charge generation layer and a protective layer, which are successively overlaid on a support, containing a particular anti-oxidizing agent either in the charge generation layer or in the protective layer are proposed as described in Japanese Laid-Open Patent Applications 63-44662, 63-50848 to 63-50851, 63-52146 and 63-52150, which are capable of preventing the deterioration of the electric chargeability of the photoconductors resulting from the generation of ozone by the anti-oxidizing agent.
In these electrophotographic photoconductors, however, the anti-oxidizing effect of the anti-oxidizing agent does not last for an extended period of time while in use.